@fungi: I have not observed this in practice.  But ... the default config in Glance allows additional image properties; default limit on properties/image is 128; default limit on members/image is 128; default limit on tags/image is 128.  So for each row added to the images table, you could have up to 128 rows added to image_properties and image_tags.

Here's from my devstack, creating an image record with some core image properties, 128 additional properties, 128 tags:
devVM! time curl -X POST -H "x-auth-token: $TK" -d @big-image-request.json http://localhost:9292/v2/images
{"prop104": "val104", "prop21": "val21", "prop49": "val49", "prop48": "val48", "prop41": "val41", "prop40": "val40", "prop43": "val43", "prop42": "val42", "prop45": "val45", "prop44": "val44", "prop47": "val47", "prop46": "val46", "prop74": "val74", "prop75": "val75", "prop76": "val76", "prop77": "val77", "prop70": "val70", "prop71": "val71", "prop72": "val72", "prop73": "val73", "prop78": "val78", "prop79": "val79", "name": "freaking-big-image", "architecture": "frank-lloyd-wright", "container_format": "bare", "min_ram": 1024, "prop63": "val63", "prop62": "val62", "prop61": "val61", "prop60": "val60", "prop67": "val67", "prop66": "val66", "prop65": "val65", "prop64": "val64", "prop69": "val69", "prop68": "val68", "prop109": "val109", "prop108": "val108", "os_distro": "custom", "prop82": "val82", "tags": ["tag125", "tag124", "tag127", "tag126", "tag121", "tag120", "tag123", "tag122", "tag23", "tag22", "tag21", "tag20", "tag27", "tag26", "tag25", "tag24", "tag29", "tag28", "tag110", "tag111", "tag112", "tag113", "tag114", "tag115", "tag116", "tag117", "tag118", "tag119", "tag38", "tag39", "tag34", "tag35", "tag36", "tag37", "tag30", "tag31", "tag32", "tag33", "tag4", "tag5", "tag6", "tag7", "tag0", "tag1", "tag2", "tag3", "tag8", "tag9", "tag109", "tag108", "tag107", "tag106", "tag105", "tag104", "tag103", "tag102", "tag101", "tag100", "tag49", "tag48", "tag41", "tag40", "tag43", "tag42", "tag45", "tag44", "tag47", "tag46", "tag58", "tag59", "tag52", "tag53", "tag50", "tag51", "tag56", "tag57", "tag54", "tag55", "tag67", "tag66", "tag65", "tag64", "tag63", "tag62", "tag61", "tag60", "tag69", "tag68", "tag70", "tag71", "tag72", "tag73", "tag74", "tag75", "tag76", "tag77", "tag78", "tag79", "tag89", "tag88", "tag85", "tag84", "tag87", "tag86", "tag81", "tag80", "tag83", "tag82", "tag98", "tag99", "tag96", "tag97", "tag94", "tag95", "tag92", "tag93", "tag90", "tag91", "tag16", "tag17", "tag14", "tag15", "tag12", "tag13", "tag10", "tag11", "tag18", "tag19"], "prop114": "val114", "prop115": "val115", "prop116": "val116", "prop117": "val117", "prop110": "val110", "prop111": "val111", "prop98": "val98", "prop99": "val99", "prop96": "val96", "prop97": "val97", "prop94": "val94", "prop95": "val95", "prop92": "val92", "prop93": "val93", "prop90": "val90", "prop91": "val91", "prop16": "val16", "prop17": "val17", "prop14": "val14", "prop15": "val15", "prop12": "val12", "prop13": "val13", "prop10": "val10", "prop11": "val11", "checksum": null, "prop18": "val18", "prop19": "val19", "prop4": "val4", "prop5": "val5", "prop6": "val6", "prop7": "val7", "prop0": "val0", "prop1": "val1", "prop2": "val2", "prop3": "val3", "prop8": "val8", "prop9": "val9", "prop112": "val112", "prop113": "val113", "prop121": "val121", "prop120": "val120", "prop123": "val123", "prop122": "val122", "prop89": "val89", "prop88": "val88", "prop85": "val85", "prop84": "val84", "prop87": "val87", "prop86": "val86", "prop81": "val81", "prop80": "val80", "prop83": "val83", "size": null, "virtual_size": null, "prop118": "val118", "disk_format": "raw", "prop119": "val119", "os_version": "1.2.3.4", "schema": "/v2/schemas/image", "instance_uuid": "96042ceb-6868-475c-a1cd-57d6d6cf905a", "visibility": "private", "min_disk": 1024, "prop30": "val30", "prop31": "val31", "prop32": "val32", "prop33": "val33", "prop34": "val34", "prop35": "val35", "prop36": "val36", "prop37": "val37", "prop38": "val38", "prop39": "val39", "prop105": "val105", "protected": false, "prop103": "val103", "prop50": "val50", "prop102": "val102", "prop101": "val101", "prop100": "val100", "updated_at": "2016-02-12T20:27:08Z", "file": "/v2/images/b932894f-4497-484e-84ed-3605af072f52/file", "id": "b932894f-4497-484e-84ed-3605af072f52", "prop29": "val29", "prop28": "val28", "prop27": "val27", "prop26": "val26", "prop25": "val25", "prop24": "val24", "prop23": "val23", "prop22": "val22", "owner": "9beb1ae94c0f46318dd4bf5d94723e94", "prop20": "val20", "prop107": "val107", "status": "queued", "prop106": "val106", "prop58": "val58", "prop59": "val59", "prop52": "val52", "prop53": "val53", "created_at": "2016-02-12T20:27:08Z", "prop51": "val51", "prop56": "val56", "prop57": "val57", "prop54": "val54", "prop55": "val55", "self": "/v2/images/b932894f-4497-484e-84ed-3605af072f52"}
real	0m0.580s
user	0m0.004s
sys	0m0.004s

So it looks like you can create a lot of images pretty quickly.