It seems to be an issue with the starting code. Just submitting the "solution" without making any changes yields:
tests/Fixture.cs(10,38): error CS0103: The name 'MorseCodeDecoder' does not exist in the current context
tests/Fixture.cs(10,68): error CS0103: The name 'MorseCodeDecoder' does not exist in the current context

So, the test is actually expecting MorseCodeDecoder, not Decoder.

There is a pause of 1 time unit between characters. So, one dash would look like "111", 3 dots would look like "10101".
Pauses between words have 3 time units difference. So the string "e e" would look like "10001".
You can assume that there is no message only containing a single character (my solution would have broken if it had that, and it was accepted).

I have the same problem, with the same exact numbers (1220 and 1760).

So, either we made the same error, or the test is wrong.

all well and good, but only 3 things are mentioned in the requirements:

• Optimize the given Brainfuck code.
• Check it for mistakes.
• Translate the given Brainfuck programming code into C programming code.

If there is a hidden requirement, such as "and it should be able to process 90k input in under 12 seconds", that should at least be mentioned. Furthermore, the time you get seems to be different per programming language (20 seconds for C++). This makes it a rather unbalanced problem IMHO.

I guess you can expect the participant to create efficient code, it is 2kyu after all. But this requirement seems too well hidden, and too random for me.

In the Python solution, I keep getting a timeout, saying my code takes over 12 seconds to complete.

Now, my code may not be optimal, but on my run-of-the-mill desktop machine, even the code sample at https://pastebin.com/SaU5Jfcv (mentioned in the comment from Naiten) completes in under a second.

For C++, the boilerplate says:
class PiApprox
{
public:
static string iterPi(double epsilon);
};

Which is incorrect without "using namespace std".

Please change to
class PiApprox
{
public:
static std::string iterPi(double epsilon);
};