The framerate of the GB and GBA is 262144/4389 frames per second (roughly 59.7275), give or take clock inaccuracy.

The framerate of the DS and 3DS is 16756991/280095 frames per second (roughly 59.8261), give or take clock inaccuracy.

I'm mostly posting this for future reference since I have to look up the numbers too frequently.

@endrift Wait, so, when the DS and 3DS are in GBA backwards-compatibility mode, do they downclock slightly or are the games just a tad too fast?
@Jhynjhiruu I'm not sure but I think they run slightly too fast
@Jhynjhiruu That's roughly 0.165% too fast though so it would really only matter for speedruns
@endrift And I suppose it'd be pretty easy to correct the times automatically even then
@Jhynjhiruu hopefully the different games and saves don't affect this but I'll let you know in an hour if the copy of Ruby is a few seconds behind
@Jhynjhiruu Slightly less than 6 seconds per hour. That's actually enough to verify by leaving the same game on two different systems and watching the attract mode to see if they desync
@Jhynjhiruu It was pointed out to me that I'm using the DS frame timings in that calculation. If I use the GBA frame timings we wind up with 16756991/280896 frames per second, which is roughly 59.6555. Slower than a GBA.
@Jhynjhiruu That's about 4 1/3 seconds per hour. I have been informed by this same person that the GBA Pokémon speedrunners adjust times based on this factor now too, so yeah. Just a tiny bit slower.
@endrift Right, makes sense that they would know already