thanks for your answers, I very much appreciate the support.

It is correct, that I do not want to get rid of the sorting files, I just want them to stay in RAM where I suspect that it could be advantageous for performance.
Unfortunately I can't tell you what specific hard drives are in use because I can't access them, but I was told that they are commercial HDD (not SSD) .
Raid is most certainly employed, which one I do not know.

The Database File as well as Firebird DBMS are installed on the same machine. This server is exclusively for the Firebird DBMS + Databases, there is no other software trying to contest resources.
An ERP Software which is installed on an application-server connects to the Firebird-DB-Server using port 3050. Said application is made available to many clients using Windows RDP/ RDS.
Windows RDP/ RDS is not installed on the application server but on dedicated RDP/ RDS servers.
I have not analysed the queries because I have not written them and lack the necessary tools (and know-how). Query analysis is an issue that would require a Firebird specialist.
My hope was, that by following the performance guides, notable performance increases may be archived.
Optimal results can only be realized with an in-depth analysis, I am very aware of that but I would like to get the "simple tuning" right first.
Alexey told me, that the TempCacheLimit can be set to 20 GB.
My current setting is 567108864 (540 MB), that's much too low, I will increase it.
 
After reading all the FB guides online I tried to optimize the Firebird.conf to the best of my ability (see below).
Maybe someone will spot a bad setting and give me a tip :-)

 #RootDirectory =

#DatabaseAccess = Full

#ExternalFileAccess = None

#UdfAccess = Restrict UDF

#TempDirectories =

#LegacyHash = 1

#Authentication = native

#AuditTraceConfigFile =

#MaxUserTraceLogSize = 10

DefaultDbCachePages = 2048

#DatabaseGrowthIncrement = 134217728

#FileSystemCacheThreshold = 65536

#FileSystemCacheSize = 0

#RemoteFileOpenAbility = 0

TempBlockSize = 2048576

TempCacheLimit = 567108864

#CompleteBooleanEvaluation = 0

#DeadlockTimeout = 10

#MaxUnflushedWrites = 100

#MaxUnflushedWriteTime = 5

#BugcheckAbort = 0

#OldColumnNaming = 0

#OldSetClauseSemantics = 0

#RelaxedAliasChecking = 0

#ConnectionTimeout = 180

#DummyPacketInterval = 0

#RemoteServiceName = gds_db

#RemoteServicePort = 3050

#RemoteAuxPort = 0

#TcpRemoteBufferSize = 8192

#TcpNoNagle = 1

#RemoteBindAddress =

LockMemSize = 99614720

#LockGrantOrder = 1

#LockAcquireSpins = 0

LockHashSlots = 30011

#EventMemSize = 65536

#CpuAffinityMask = 1

#UsePriorityScheduler = 1

#PrioritySwitchDelay = 100

#PriorityBoost = 5

#GCPolicy = combined

#GuardianOption = 1

#ProcessPriorityLevel = 0

#IpcName = FIREBIRD

#RemotePipeName = interbas

#Redirection = 0

Best Regards,
James