Its normal, and annoying. Easy fix, change out your turn signal timer module with a solid state one. That way your electrical load on the car wont determine the light blink speed. I did this fix because when having a trailer hooked up the extra electrial load made my lights slow.
The part should be about 10-12 bucks from local autoparts store. Easy change out, 10 min tops, take of kick panel for driver side, near center of dash you'll see a tiny aluminum can shaped device with two wires going into it, thats the signal clicker. Its not screwed/bolted in so pull it off the bracket thats grasping it, yank off the connector and connect your new solid state clicker.
1999-2007: ___1988 Bonneville LE LN3 (Vin C) @ 273,000 miles
and still going strong (<-Faded sun fried grayish ie needs paint)
2007-Present: ___1993 Bonneville SE L27 @ 210,000
Speeding around So Cal (<-Beautiful Forest Green)
Mods: Sylvania Silver Star headlamps. Remote start system with pageing device. K&N air filter. Tranny Cooler. High-flow cat with Magnaflow muffler. Full Class II hitch setup.