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IVs make a huge difference.
For perspective:
A jolly Salamence with 0 speed IV and 252 speed EVs will only reach 289 speed. This means that anything with 85 base speed can outspeed it.
You might be able to get by with an IV of 28~ if your other stats are good, but there are some stats(ex: speed on a sweeper, hp for substitute users) that you absolutely cannot skimp on.