Slides on the TT system. They have gotten up to capacity on the The Taniwha Tubes and they are almost always "Ride Now". And The Maku Pui slides are also "Ride Now" for much of the day even on capacity days. And Ika Moana usually has little to no line. Although Honu is quite popular and builds a wait early.
There are 2 design issues that ProSlide are working on that are sort of crippling the park. I'll cover the bigger one first. The 2 serpentine drop slides are the tallest that ProSlide has ever built, and to conform to the layout around the volcano, the ride path, in places, is quite far from the center support structure. They have found that with riders upwards towards 300 lbs. the lateral g's are flexing the supports more than ProSlide is comfortable with. They have reduced the weight limit on these 2 slides to 200 lbs. until they can get beefier supports in the couple of places that are an issue. This is seriously effecting capacity at the drop slides, because everyone over 200 lbs. is queuing for the red straight drop slide causing huge wait times, or just heading to other attractions over loading them. This will be fixed, but it is going to take a little while. Until it's fixed, those under 200 lbs. will find almost no wait on the 2 serpentine slides. So there is that plus.
The second design issue that is hurting capacity is on the Punga Racers mat slides. It seems the angle of entry into landing pool is off, resulting in people being sent tail over teacup if they don't pull back on the mat as they head into the landing pool. It is pretty bad on 2 of the 4 slides, so they are only operating 2 slides until the fix can be made. This is an issue because the mat slides are designed to be one of the parks high capacity attractions and right now they are sort of limping along with 2 slides.
All that said, significant improvements have been made. Even on capacity days (which they all have been since it opened), there usually is at least one set of slides that are "ride now". And yes, you can ride the "ride now" slides while you are waiting in the virtual queue for another attraction.
They are also bringing a lot more of the interactive elements around the park online giving people more things to do and explore while waiting for their Tapu Tapu times.
And concerning capacity, the parks caps at 6000. Typhoon is 6000, BB is 5000.