This is the eternal question, theoretically they affect flow, though it all, many people treat them as "demons" based on urban myths mostly: "they're cutting it down to 50%" or others. The only way to get it tested it's to use a flow meter, cause the flow is affected for a lot of things, not just the elbows, also the loop distance and curves (or kinks), the loop fluid (density and viscosity) as well as the position of water elements as blocks or radiators. Once said that, of course 90 degree elbows are gonna affect flow but the amount of it it's gonna change from one loop to another. There are math and physic formulas that can help you with that but the easiest one is to measure it by yourself in your specific system. In my personal one, i can see how fast the liquid is moving (using Mayhem Aurora fluid) and as far as i can see, the shortness of my circuit, the fact that no tube is kinked and a powerful pump is giving me almost the same speeds on every part of the loop w/o noticeable flow restrictions. Also you can find 90 degree rotary elbows which are carrying a 45º inside as some Bitspower fittings. Yeah, i'm using them in two critic and very limited points of my circuit: the return from radiator to reservoir which will cause a curve between these elements and will disturb the res-pump tube and could move the position of my radiator, also the pump-block connection, which i decided to make it straight to allow more air to pass through the MB and the plate of the GFX. It was just to maximize the free space to maintain the circuit in case any emergency happens and also some fresh inners.