Does iphone become slower everytime a new one releases?

For example the iphone 5s would get slower after 6. Iphone 6 gets slower after 6s. Iphone 6s would get slower after the newest one, iphone SE.

or does it take about 2 phone releases to get slow?

Yes and magic fairy dust falls from your pillow

This is a common scam to get you to buy a new phone.

Your phone will get slower over time as newer versions of IOS are released. This is because every new version requires more resources from the CPU/GPU to run it. Apple tends to take it to the extreme and supports phones for a very long time. However phones like the iPhone 4S are very slow running IOS 9. The chipset just isn't really that great at running such a large OS. Eventually that will happen with the 6S too when IOS 13 and 14 releases.

This is exactly why Google recommends that Android phones in comparison are only updated for 2 years after release. (Almost all manufacturers abide by this recommendation.) This avoids such such a situation. This way the phone won't get stuck running such an advanced future version of Android that requires so much from the chipset to run. This is why that my Galaxy S4 still runs just as fast as the day I bought it. It came with Android Jelly Bean and was removed from Android update support running Lollipop. Lollipop runs just as fast and actually a little faster than the older version. But continuing support would then start to have the opposite effect going forward down the line as the years roll by.