SOLR project stories. Lack of SOLR post filter support

For past few quarters, I am working on a project to implement security on object documents. My goals is Decorate every SOLR document with ACL field. This ACL field used to determine what users have access to this document. ACL syntax is something like +u (dave)-g(support) etc. My thoughts are process these ACL fields after search aka I want to subject the query component results via some kind of post filter. However SOLR is not offering any direct mechanism to specify this kind of post filter along with the search request. At Lucene level, there is an option to specify the filter however current AS IS implementation, it sucks.
It iterates entire document sets. For large documents this sucks. Also we need to SOLR distributed capabilities. Also during computing the ACL fields, I tried to encode users names etc with Base64, URLEncoder.encode etc. For small set of strings, this is working Ok. But for large sets, it is a pain. Ultimately affecting the search performance.
Another blocker.
Encode/decoder tes code.
startTime = System.currentTimeMillis();
String inputText = "Hello#$#%^#^&world";
for (int i =0;i<50000;i++)
{
String baseString = i+ " "+inputText;
encodedText = URLEncoder.encode(baseString,"UTF-8");
decodedText = URLDecoder.decode(encodedText, "UTF-8");
}
endTime = System.currentTimeMillis();

elapsedTime = endTime - startTime;

System.out.println( "\n URLEncoder/decoder Elapsed Time = " + elapsedTime + "ms");

>>>>
Elapsed Time = 2246ms