I guess this feedback is more of a complaint than anything, because my OBS Classic runs very smoothly, but when I put the same settings into OBS Studio, my entire laptop just starts lagging and OBS Studio only runs at like 5 fps, dropping thousands of frames per minute, and spits out high-encoding warnings and can't run at all. I even tried lowering the bitrate to about 500 (on OBS Classic I can run it on 1,500 no problem), fps to 30 (Classic can do 60 no problem), lowered the output resolution to all of the possible ones, played around with all the downscale filters, as well as the CPU usage preset speeds, and all of the possible combinations thereof. Studio just will not run properly. But with Classic, everything on the highest settings I can run them at streams pretty much perfectly.
I can't read code so I can't comment on the development of OBS, but my best guess is that the newer OBS Studio is designed to be more functional with newer, higher end machines built for streaming, whereas the older OBS Classic may not be as powerful in terms of features and higher quality, but it allows to be run on older, less powerful machines smoothly. (Since my laptop does not have a graphics card and only has onboard graphics, which may not be ideal for streaming, but nonetheless it works nearly perfectly on OBS Classic.)
I can't read code so I can't comment on the development of OBS, but my best guess is that the newer OBS Studio is designed to be more functional with newer, higher end machines built for streaming, whereas the older OBS Classic may not be as powerful in terms of features and higher quality, but it allows to be run on older, less powerful machines smoothly. (Since my laptop does not have a graphics card and only has onboard graphics, which may not be ideal for streaming, but nonetheless it works nearly perfectly on OBS Classic.)