The cost of moving 4 KB is miniscule. Assume a anemic basic desktop with a 2 GHz clock and 2 instructions per clock. You would be able to issue 4 billion 8-byte stores per second resulting 32 GB/s or 32 bytes per nanosecond. Memory bandwidth is going to be on the order of 40-60 GB/s for a basic desktop, so you will not run into memory bandwidth bottlenecks with the aforementioned instruction sequence. So, 4 KB of extra stores is a grand total of 128 additional nanoseconds.

In comparison, the average context switch time of Linux is already on the order of ~1,000 nanoseconds [1]. We can also see additional confirmation of the store overhead I computed as that page measures ~3 us for a 64 KB memcpy (load + store) which would be ~187 nanoseconds for 4 KB (load + store) where as the context saving operation is a 2 KB register -> store and a 2 KB load -> register.

So, your system call overhead only increases by 10 percentage points. Assuming you have a reasonably designed system that does not kill itself with system call overhead, spending the majority of the time actually handling the service request, then it constitutes a miniscule performance cost. For example, if you spend 90% of the time executing the service code, with only 10% in the actual overhead, then you only incur a 1% performance hit.

[1] https://eli.thegreenplace.net/2018/measuring-context-switchi...