It sounds like you are taking the proper corrective actions to bring the vehicle back to life after a period of deferred maintenance. Good for you. The rpm thing, where it goes up but the vehicle speed doesn’t correspond, that’s a transmission problem, assuming you’ve got a conventional automatic. A proper service can sometimes fix that, which usually means to drop the pan, clean the bottom of the pan, sieve the old fluid for metal debris, new transmission filter, and refill with the proper fluid. If that seems to help doing it one time, drive a hundred miles or so, then try the same thing again. You could get lucky after repeating that a time or two, and the transmission slipping problem would be fixed.
If you feel the engine simply lacks power, with no check engine light and no diagnostic codes, and with a basic tune-up already done, start with these checks
- engine compression ok?
- engine air filter clean? bird nest or plastic bag stuck anywhere in cold air intake path?
- any signs of cat problems? e.g. higher than normal intake manifold vacuum levels?
- ignition base timing and advance curve check out ok?
As far as sludge, removing the valve cover is a good way to check for that. Often you can peek inside that area with a flashlight and a dental mirror just by removing the oil filler cap, so try that first. If you remove the valve cover, and that’s never been done before, best to install a new gasket. Check the factory service data as there may be a requirement for rtv placement at certain spots as part of the gasket replacement. Striclty follow the service data on the order to tighten the valve cover bolts , and the proper torque. Overtightening those be bolts is a common diy’er error, and can be an expensive one to make.
If the engine oil and filter has been changed every 6K miles, I doubt you’ll have much problem with sludge causing any problems, unless you have a variable valve timing engine.