Since you can create memory objects in your programs,
you must also be able to delete them. For table
references or string references, use the ABAP statements
CLEAR, FREE, and REFRESH.1 Applying
these statements to a reference variable pointing to a
string or table body that is also referenced by other
reference variables (in other words, to a memory
object that is shared by multiple references) leaves the
memory object alive, decrements its reference count
by one, and sets the reference variable to the typespecific
initial value. Memory will only be freed once
the table body or string is no longer shared by other
references (or, in other words, once the reference
count is one) when the delete operation occurs. A
memory leak simply cannot occur as long as you
remember to clear all internal tables or string references
once they are no longer used.
...
Low used-to-allocated memory ratio: You can
create ABAP internal tables with an arbitrary size
by using the INITIAL SIZE parameter. Using this
parameter carelessly can lead to wasted memory
if the application can’t ever fill the allocated size.
Also remember that the ABAP runtime environment
automatically increases the allocated memory
for an internal table as needed when rows are
added, but never decreases the allocated memory,
even if all the rows have been deleted. For this
reason, internal tables in the root set that are only
used during the early stages of an application are a
potential source of wasted memory. The system
can’t reuse the allocated storage until the table
bodies are deleted.

как это можно ускорить???

Internal tables are a dynamic data structure. Their memory requirements are met in blocks. The initial memory allocation (hereafter called the OCCURS area), can be controlled using the " OCCURS n" or "INITIAL SIZE n " addition in the table definition (see DATA, TYPES). Once the OCCURS area is full, the next block to be created is twice as big as the OCCURS area (as long as this is not greater than 8 KB). All further blocks are then created with a constant size of 12 KB.

You can leave it to the system to determine the size of the OCCURS area by specifying n = 0. In this case, the system allocates only a "small" portion of memory at the first INSERT or APPEND statement. "OCCURS 0" or "INITIAL SIZE 0" means that 16 <= n <= 100 (depending on the line width).

It only makes sense to specify a concrete value of n > 0 when you know exactly how many entries the table will have, and you want to set up the OCCURS area exactly. This can be particularly important if you want to nest internal tables (where an "outer" internal table contains one or more other internal tables in each line, and the "inner" tables only have a few entries (no more than 5, for example).

To avoid excessive memory requirements, the system handles large values of n as follows: The largest possible value of n is n_max = 8 KB divided by the line width. For larger values, n is set such that n multiplied by the line width is around 12 KB.