When i drag 'n drop/copy, lots of big and small files the transfer speed gets reduced,compared to the speed i get when transfering the files one by one,its that a regular issue with Mechanical HDDs?,could the use of an SSD improve the Performance?
Do you mean that you somehow start multiple simultaneous copies? If so, there is every reason to expect that it should be less efficient than serial copying.
First, if you assume that the disk is infinitely fast, then there will be the system overhead of context switches between multiple copying processes. In extreme cases you could start enough copying processes that they could not all fit in core at once and then you'd have to add virtual memory swapping to the system's overhead.
But disks are not infinitely fast, and gathering multiple files as requested by multiple processes will likely require drive seeks which are the slowest operation on a mechanical drive. The kernel does try to re-order drive accesses to minimize the head seeks required, but this is difficult to make optimal as the system can't predict what will be requested in the future.
Finally, the system has to protect certain data structures (e.g. the free block list) from being manipulated by multiple processes concurrently which it does by blocking processes P2, P3, P4... Pn while P1 is allocating space.
Because your question is ambiguous, another answer is that if you select a bunch of files in nautilus and copy them elsewhere with a single drag-drop, nautilus only creates a single process to produce the copies sequentially. In this case a faster drive would indeed be faster; the question then becomes how much fast can you afford?
It's possible that your files are located at different portions of the disk, causing it to have to seek to various cylinders and tracks on the disk surface to find the data. An SSD would improve your performance in that case, by eliminating the physical seeking. However, it can also be possible that when you copy large numbers of files the OS schedules the copy process as a lower priority process, particularly if you are running other I/O heavy processes.