Trust me, 45 FPS does not feel like 15 FPS - 45 FPS should be playable but 15 FPS is awful quit-the-game bad. 45 FPS is obviously not as bad as 15 FPS but probably looks bad because the frames are irregularly updating. Since the screen updates at 60 Hz, a game drawing at 45 FPS will be drawing a frame every 1.5 screen refreshes, causing an ugly pattern like: draw, skip, draw, draw, skip, draw, skip, draw, draw, skip. On the other hand 15 FPS would be draw, skip, skip, skip, draw, skip, skip skip, draw...
Irregular frames don't look great but provides a less laggy gameplay since you are getting more frames and can respond more quickly to gameplay events. We previously had an experimental feature to drop the framerate to a regular 30 FPS, but it didn't seem possible to implement reliably (it still had ugly irregular framerates and worked differently on different browsers) so we gave up on it.